THS4541-Q1 Datasheet, PDF (39/60 Pages) Texas Instruments – 850-MHz Fully Differential Amplifier

www.ti.com
THS4541-Q1
SLOS930A – NOVEMBER 2015 – REVISED NOVEMBER 2015
These equations and design flow apply to any FDA. Using the feedback resistor value as a starting point is
particularly useful for current-feedback-based FDAs such as the LMH6554, where the value of these feedback
resistors determines the frequency response flatness. Similar tables can be built using the equations provided
here for other source impedances, Rf values, and gain ranges.
Note the extremely low Rg1 values at the higher gains. For instance, at a gain of 14 V/V, that 7.32-Ω standard
value is transformed by the action of the common-mode loop moving the input common-mode voltage to appear
like a 50-Ω input match. This active input impedance provides an improved input-referred noise at higher gains;
see the Noise Analysis section. The TINA model correctly shows this actively-set input impedance in the single-
ended to differential configuration, and is a good tool to validate the gains, input impedances, response shapes,
and noise issues.
9.4.1.4 Input Impedance for the Single-Ended to Differential FDA Configuration
The designs so far have included a source impedance, Rs, that must be matched by Rt and Rg1. The total
impedance at the junction of Rt and Rg1 for the circuit of Figure 63 is the parallel combination of Rt to ground,
and the ZA (active impedance) presented by Rg1. The expression for ZA, assuming Rg2 is set to obtain the
differential divider balance, is given by Equation 11:
§
¨¹
Rg1
·§
¸¨¹
Rf
·
¸
ZA Rg1^©
Rg2 _¹© Rg1_¹
2 ^Rf
Rg2
(11)
For designs that do not need impedance matching, but instead come from the low impedance output of another
amplifier for instance, Rg1 = Rg2 is the single-to-differential design used without an Rt to ground. Setting Rg1 =
Rg2 = Rg in Equation 11 gives the input impedance of a simple input FDA driving from a low-impedance, single-
ended source to a differential output as shown in Equation 12:
1 ^Rf
ZA
2Rg
2
Rg
Rf
Rg
(12)
In this case, setting a target gain as Rf / Rg ≡ α, and then setting the desired input impedance, allows the Rg
element to be resolved first, and then the required Rf to get the gain. For example, targeting an input impedance
of 200 Ω with a gain of 4 V/V, Equation 13 gives the physical Rg element. Multiplying this required Rg value by a
gain of 4 gives the Rf value and the design of Figure 72.
Rg ZA ^{2 D}
21 D
(13)
THS4541 Wideband,
Fully-Differential Amplifier
200- Input Impedance,
Gain of 4 V/V Design
Rg1
120
+
Vs
±
Vocm
Rf1
480
Vcc
±
+
FDA
±
+
PD
Rg2
Vcc
120
R1
500
Output
Measurement
Point
Rf2
480
Figure 72. 200-Ω Input Impedance, Single-Ended to Differential DC-Coupled Design with Gain of 4 V/V
Copyright © 2015, Texas Instruments Incorporated
Product Folder Links: THS4541-Q1
Submit Documentation Feedback
39